MRI is a medical imaging technique based on the magnetization properties of atomic nuclei. During an MRI imaging procedure, a magnetic field and a pulse of radio frequency (RF) energy are applied to a target such as a living subject or tissue specimen to produce an image used for imaging internal biological structures. The applied magnetic field aligns the protons that are normally randomly oriented within the water nuclei of the target being examined. This alignment is then perturbed by the applied RF pulse energy, such that the nuclei return to their resting orientations through various relaxation processes, and thereby emit RF energy which is measurable. For example, the emitted RF energy is measured according to certain time periods following the applied RF pulse. Temporal parameters, including repetition time (TR) and echo time (TE), associated with the temporal sequence of RF pulses applied and the collection of echo signal following an initial excitation pulse can be varied to create different types of MR images. Repetition time is the amount of time between successive RF pulse sequences applied to the same region of the target (e.g., same volume slice), and echo time is the time between the RF pulse delivery and the receipt of the echo signal. The measured data is processed using signal processing techniques to produce the MR images, e.g., including Fourier transformation to convert the frequency information contained in the measured signals from each location in the imaged plane to corresponding intensity levels, which are then displayed as shades of gray in a matrix arrangement of pixels.
MRI is based on the property of nuclear magnetic resonance (NMR). NMR is a physical property in which the nuclei of atoms absorb and re-emit electromagnetic energy at a specific resonance frequency in the presence of a magnetic field. The absorption and reemission of energy can be dependent on the strength of the magnetic field and the magnetic property of the atoms (e.g., atoms whose nuclei possesses magnetic spin).